The invention provides a process for preparing diaryl carbonates from dialkyl carbonates and aromatic hydroxyl compounds using at least two reaction columns, a process section for recovering the dialkyl carbonate used in the reaction and for removing the alcohol of reaction, one or more process steps for removing the by-products obtained in the process which have a boiling point between that of the dialkyl carbonate and that of the alkyl aryl carbonate formed during the preparation of the diaryl carbonate, and a process step for further purification of the diaryl carbonate obtained from the reaction columns.
The preparation of aromatic and aliphatic-aromatic carbonic esters (carbonates) by transesterification proceeding from aliphatic carbonic esters and aromatic hydroxyl compounds is known in principle. This is an equilibrium reaction wherein the equilibrium position is shifted almost entirely in the direction of the aliphatically substituted carbonates. It is therefore comparatively easy to prepare aliphatic carbonates from aromatic carbonates and alcohols. In order, however, to carry out the reaction in the reverse direction towards aromatic carbonates, it is necessary to shift the very unfavourable equilibrium effectively to the side of the aromatic carbonates, for which not only very active catalysts but also suitable process regimes have to be employed.
It is known that such equilibrium reactions can be carried out in columns and they can be shifted in this way advantageously in the direction of the desired product formation (e.g. U. Block, Chem.-Ing. Techn. 49, 151 (1977); DE-A 38 09 417; B. Schleper, B. Gutsche, J. Wnuck and L. Jeromin, Chem.-Ing.-Techn. 62, 226 (1990); Ullmans Encyclopädie der techn. Chemie, 4th Ed., vol. 3; p. 375 ff. 1973).
In the known processes, the transesterification is therefore also preferably effected continuously in a countercurrent transesterification in one or more reaction columns.
The processes known from the literature, for example EP-A 461 274, DE-A 42 26 755, DE-A 42 26 756, however, generally describe only those process steps in which the reaction to give the diaryl carbonate takes place by transesterification and/or disproportionation. WO-A 2006/033291, EP-A 1 775 280, EP-A 1 767 516, EP-A 1 767 517, EP-A 1767 518, EP-A 1 762 559 and EP-A 1 762 560 additionally give hints with regard to the apparatus configurations of reaction columns for preparation of diaryl carbonates. For the economic viability of a process, however, not just the process sections in the region of the reaction but, in some cases to a much greater degree, the subsequent steps for workup are of relevance. The literature to date contains only very little information on this subject.
Since the preparation of diaryl carbonates by reaction of an aromatic hydroxyl compound with a dialkyl carbonate, as experience has shown, is energetically very demanding, measures for reducing the energy consumption likewise play an important role. The literature currently available also gives very little information on this subject.
EP-A 781 760 describes a continuous process for preparing aromatic carbonates by reacting a dialkyl carbonate with an aromatic hydroxyl compound in the presence of a catalyst, continuously removing the aromatic carbonate formed in the reaction, the alcoholic by-products, the dialkyl carbonate and the aromatic hydroxyl compound, the dialkyl carbonate and the aromatic hydroxyl compound being recycled back into the reaction. However, it is not described how the dialkyl carbonate used in the reaction is separated from the alcoholic by-product (alcohol of reaction). However, experience has shown that this step is very energy-intensive and demanding, especially when dialkyl carbonate and alcohol of reaction are difficult to separate from one another. Moreover, the isolation of the diaryl carbonate after the reaction is not described either, but, owing to the high purity demands thereon, is very complicated. Moreover, no statements regarding possible energy savings are made.
EP-A 1 638 917 describes a process for recovering a product from a waste stream by contacting with an alkyl alcohol, the product recovered comprising diaryl carbonate, aromatic alcohol, alkyl salicylate and alkyl alcohol. One disadvantage of the process described is that the reaction is effected in three stages, which makes it very complicated. Another is that high-boiling waste streams are obtained at two points. Removal of the catalyst before the isolation of the diaryl carbonate gives rise to the first waste stream, and the subsequent workup consisting of two distillation columns to the second waste stream. The workup for isolation of the diaryl carbonate is thus very demanding both in apparatus and energetic terms. In addition, the quality of the diaryl carbonate thus prepared at 99.5% by weight is very poor and suitability for the preparation of polycarbonate is questionable. The separation of the mixture of alcohol of reaction and dialkyl carbonate obtained in the reaction is not described either.
WO-A 2005/1000776 describes a process for preparing an alkyl aryl ether which is formed in the reaction of a dialkyl carbonate with an aromatic hydroxyl compound. In this process, diaryl carbonate is additionally also obtained. The process structure comprises three reaction columns and two further distillation columns for the purpose of isolating the alkyl aryl ether. The fact that a controlled purification of the alkyl aryl ether is an aim in the process described here leads to the conclusion that the amount formed in the reaction is high. In the preparation of diaryl carbonates, however, the recovery of a high-purity alkyl aryl ether is not first priority, and the aim is instead minimum formation of this by-product obtained in the transesterification. Moreover, the reaction regime comprising three reaction stages is very complicated, and no statements are made regarding the workup of the diaryl carbonate and the separation of the mixture which is obtained in the reaction and comprises dialkyl carbonate and alcohol of reaction. EP-A 1 237 842 also describes a comparable process, and therefore the disadvantages already mentioned likewise apply to this.
WO-A 2004/016577 describes a process for preparing aromatic carbonates from dialkyl carbonate and an aromatic hydroxyl compound in the presence of a catalyst in a plurality of separate and series-connected reaction zones of a reactor arrangement, wherein the heat of condensation obtained in the condensation of the vapour stream of the last reaction zone is used to heat the liquid stream introduced into the first reaction zone. However, a disadvantage of this process is the complicated reactor arrangement. In addition, the energetic integration of this process is in need of improvement and is limited only to the process section of reaction. Subsequent steps for the workup are not described.
JP-A 2002-020351 describes a batchwise process for preparing diaryl carbonate, from which heat can be utilized for steam raising. However, disadvantages of this process are the batchwise performance and the reactor arrangement used for the reaction, which has an attached distillation column. Subsequent steps for the workup are not described.
There was accordingly still a need to provide a process for preparing aromatic carbonates, preferably diaryl carbonates, which includes a workup of product and waste streams, which does not have the disadvantages specified above and in which, compared to the known processes specified above, energy integration is possible in an efficient manner and improved energy integration can be achieved.
The object on which the invention was based accordingly consisted in providing a process for preparing aromatic carbonates, preferably diaryl carbonates, which includes a workup of product and waste streams and in which, compared to known processes, energy integration is possible in an efficient manner and improved energy integration can be achieved.